1. Field of the Invention
The present invention relates to a method and apparatus for detecting a surface of a liquid in a container and, more particularly, to a method and apparatus for use in an automated blood/plasma sampling system for detecting a surface of a liquid in a container and for controlling a position of a probe with respect to the surface of the liquid.
An example of an automated blood/plasma sampling system for which the present invention is applicable is disclosed, for example, in U. S. patent application Ser. No. 07/833,951 to Hulette et al., filed Feb. 11, 1992, now abandoned, which is a continuation-in-part application of U.S. patent application Ser. No. 07/443,951, filed Dec. 1, 1989, now abandoned. The subject matter of the continuation-in-part application to Hulette et al. is incorporated herein by reference. An example of a piercing and sampling probe is disclosed in U.S. patent application Ser. No. 07/874,371 filed Apr. 27, 1992, and now U.S. Pat. No. 5,354,537 by Moreno. The subject matter of the application to Moreno is incorporated herein by reference. Another example of a sampling probe is disclosed in U.S. Pat. No. 5,178,019, issued Jan. 12, 1993, to Keiter. The subject matter of the patent to Keiter is incorporated by reference herein. An example of a memory control device associated with a robotic arm controller is disclosed in a U.S. patent application Ser. No. 08/088,550 to Ramey filed concurrently with the present application and and now abandoned but continued as application Ser. No. 08/404,121 filed Mar. 14, 1995. The subject matter of the application to Ramey is incorporated by reference herein. The subject matter of the applications to Hulette et al., to Moreno and to Ramey, and the patent to Keiter are each assigned to the same assignee as the present application.
2. Description of the Related Art
Automated sample handling systems are known which automatically dispense fluid samples, such as blood plasma and reagents, into a reaction well of a cuvette. Such instruments are useful in the field of biochemical analysis for measuring blood clotting times and for automatically carrying out other bioassays. Additionally, these instruments are useful in the field of chemical assays for automatically carrying out chemical assays. An automated sample handling system for carrying out blood and plasma bioassays is described in U.S. patent application Ser. No. 07/443,951 to Hulette et al.
In this particular system, fluid samples, such as blood or plasma, are stored in containers, such as test tubes, which are vacuum sealed by way of a rubber septum that must be pierced in order to withdraw a measured amount of the sample for testing purposes. U.S. patent application Ser. No. 07/874,371 to Moreno, incorporated herein by reference, discloses an example of a piercing and sampling probe suitable for piercing and sampling a measured amount of liquid.
The Hulette et al. system also includes a temperature controlled housing provided for storing fluid samples and reagents at a relatively cool temperature for preventing degradation of the samples and reagents prior to sample analysis. The temperature controlled housing typically maintains the fluid samples and reagents at a temperature of 10.degree. C. The actual analyses are generally carried out at 37.degree. C. (98.6.degree. F.), standard human body temperature. Accordingly, it is necessary to heat the fluid sample and reagents to 37.degree. C. prior to analysis. U.S. Pat. No. 5,179,019 to Keiter, incorporated herein by reference, discloses a sample probe device useful for heating fluid samples and reagents prior to analysis.
The piercing and sampling probes in the Hulette et al. system are raised and lowered in operation by a robotic arm which maneuvers a probe between reagent containers and a reaction cuvette for automatically aspirating and dispensing reagents. The surface of a liquid, whether a sample or reagent, is detected for accurately controlling movement of the probe. Basically, the surface of the liquid is detected by detecting a change in capacitance of the probe with respect to the chassis of the automated blood/plasma sampling system.
Presently available liquid surface sensing devices supplied by CAVRO Scientific Instruments, Inc., Sunnyvale Calif., do not accurately sense small volumes of liquid while at the same time being of such small size for readily mounting on a movable rack assembly holding a sampling probe. A high sensitivity CAVRO liquid surface sensing device can detect 10 .mu.l of a 1:10,000 dilution of normal saline when tested in a 10.times.75 mm glass tube. Another liquid level sensing device manufactured by Hamilton, Reno, Nev., also does not accurately sense small volumes of liquid while having a size which lends itself to mounting on a movable rack assembly holding a sampling probe. The present invention reliably senses the surface of a small volume of liquid, for example, liquid volumes less than 300 .mu.l of saline in a plastic 3 ml reagent bottle, even while piercing a sealing septum.